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 <head>
  <meta content="text/html; charset=utf-8" http-equiv="Content-Type">
  <meta content="width=device-width, initial-scale=1, shrink-to-fit=no" name="viewport">
  <title>P0690R0: Tearable Atomics</title>
<style data-fill-with="stylesheet">/******************************************************************************
 *                   Style sheet for the W3C specifications                   *
 *
 * Special classes handled by this style sheet include:
 *
 * Indices
 *   - .toc for the Table of Contents (<ol class="toc">)
 *     + <span class="secno"> for the section numbers
 *   - #toc for the Table of Contents (<nav id="toc">)
 *   - ul.index for Indices (<a href="#ref">term</a><span>, in §N.M</span>)
 *   - table.index for Index Tables (e.g. for properties or elements)
 *
 * Structural Markup
 *   - table.data for general data tables
 *     -> use 'scope' attribute, <colgroup>, <thead>, and <tbody> for best results !
 *     -> use <table class='complex data'> for extra-complex tables
 *     -> use <td class='long'> for paragraph-length cell content
 *     -> use <td class='pre'> when manual line breaks/indentation would help readability
 *   - dl.switch for switch statements
 *   - ol.algorithm for algorithms (helps to visualize nesting)
 *   - .figure and .caption (HTML4) and figure and figcaption (HTML5)
 *     -> .sidefigure for right-floated figures
 *   - ins/del
 *
 * Code
 *   - pre and code
 *
 * Special Sections
 *   - .note       for informative notes             (div, p, span, aside, details)
 *   - .example    for informative examples          (div, p, pre, span)
 *   - .issue      for issues                        (div, p, span)
 *   - .assertion  for assertions                    (div, p, span)
 *   - .advisement for loud normative statements     (div, p, strong)
 *   - .annoying-warning for spec obsoletion notices (div, aside, details)
 *
 * Definition Boxes
 *   - pre.def   for WebIDL definitions
 *   - table.def for tables that define other entities (e.g. CSS properties)
 *   - dl.def    for definition lists that define other entitles (e.g. HTML elements)
 *
 * Numbering
 *   - .secno for section numbers in .toc and headings (<span class='secno'>3.2</span>)
 *   - .marker for source-inserted example/figure/issue numbers (<span class='marker'>Issue 4</span>)
 *   - ::before styled for CSS-generated issue/example/figure numbers:
 *     -> Documents wishing to use this only need to add
 *        figcaption::before,
 *        .caption::before { content: "Figure "  counter(figure) " ";  }
 *        .example::before { content: "Example " counter(example) " "; }
 *        .issue::before   { content: "Issue "   counter(issue) " ";   }
 *
 * Header Stuff (ignore, just don't conflict with these classes)
 *   - .head for the header
 *   - .copyright for the copyright
 *
 * Miscellaneous
 *   - .overlarge for things that should be as wide as possible, even if
 *     that overflows the body text area. This can be used on an item or
 *     on its container, depending on the effect desired.
 *     Note that this styling basically doesn't help at all when printing,
 *     since A4 paper isn't much wider than the max-width here.
 *     It's better to design things to fit into a narrower measure if possible.
 *   - js-added ToC jump links (see fixup.js)
 *
 ******************************************************************************/

/******************************************************************************/
/*                                   Body                                     */
/******************************************************************************/

	body {
		counter-reset: example figure issue;

		/* Layout */
		max-width: 50em;               /* limit line length to 50em for readability   */
		margin: 0 auto;                /* center text within page                     */
		padding: 1.6em 1.5em 2em 50px; /* assume 16px font size for downlevel clients */
		padding: 1.6em 1.5em 2em calc(26px + 1.5em); /* leave space for status flag     */

		/* Typography */
		line-height: 1.5;
		font-family: sans-serif;
		widows: 2;
		orphans: 2;
		word-wrap: break-word;
		overflow-wrap: break-word;
		hyphens: auto;

		/* Colors */
		color: black;
		background: white top left fixed no-repeat;
		background-size: 25px auto;
	}


/******************************************************************************/
/*                         Front Matter & Navigation                          */
/******************************************************************************/

/** Header ********************************************************************/

	div.head { margin-bottom: 1em }
	div.head hr { border-style: solid; }

	div.head h1 {
		font-weight: bold;
		margin: 0 0 .1em;
		font-size: 220%;
	}

	div.head h2 { margin-bottom: 1.5em;}

/** W3C Logo ******************************************************************/

	.head .logo {
		float: right;
		margin: 0.4rem 0 0.2rem .4rem;
	}

	.head img[src*="logos/W3C"] {
		display: block;
		border: solid #1a5e9a;
		border-width: .65rem .7rem .6rem;
		border-radius: .4rem;
		background: #1a5e9a;
		color: white;
		font-weight: bold;
	}

	.head a:hover > img[src*="logos/W3C"],
	.head a:focus > img[src*="logos/W3C"] {
		opacity: .8;
	}

	.head a:active > img[src*="logos/W3C"] {
		background: #c00;
		border-color: #c00;
	}

	/* see also additional rules in Link Styling section */

/** Copyright *****************************************************************/

	p.copyright,
	p.copyright small { font-size: small }

/** Back to Top / ToC Toggle **************************************************/

	@media print {
		#toc-nav {
			display: none;
		}
	}
	@media not print {
		#toc-nav {
			position: fixed;
			z-index: 2;
			bottom: 0; left: 0;
			margin: 0;
			min-width: 1.33em;
			border-top-right-radius: 2rem;
			box-shadow: 0 0 2px;
			font-size: 1.5em;
			color: black;
		}
		#toc-nav > a {
			display: block;
			white-space: nowrap;

			height: 1.33em;
			padding: .1em 0.3em;
			margin: 0;

			background: white;
			box-shadow: 0 0 2px;
			border: none;
			border-top-right-radius: 1.33em;
			background: white;
		}
		#toc-nav > #toc-jump {
			padding-bottom: 2em;
			margin-bottom: -1.9em;
		}

		#toc-nav > a:hover,
		#toc-nav > a:focus {
			background: #f8f8f8;
		}
		#toc-nav > a:not(:hover):not(:focus) {
			color: #707070;
		}

		/* statusbar gets in the way on keyboard focus; remove once browsers fix */
		#toc-nav > a[href="#toc"]:not(:hover):focus:last-child {
			padding-bottom: 1.5rem;
		}

		#toc-nav:not(:hover) > a:not(:focus) > span + span {
			/* Ideally this uses :focus-within on #toc-nav */
			display: none;
		}
		#toc-nav > a > span + span {
			padding-right: 0.2em;
		}

		#toc-toggle-inline {
			vertical-align: 0.05em;
			font-size: 80%;
			color: gray;
			color: hsla(203,20%,40%,.7);
			border-style: none;
			background: transparent;
			position: relative;
		}
		#toc-toggle-inline:hover:not(:active),
		#toc-toggle-inline:focus:not(:active) {
			text-shadow: 1px 1px silver;
			top: -1px;
			left: -1px;
		}

		#toc-nav :active {
			color: #C00;
		}
	}

/** ToC Sidebar ***************************************************************/

	/* Floating sidebar */
	@media screen {
		body.toc-sidebar #toc {
			position: fixed;
			top: 0; bottom: 0;
			left: 0;
			width: 23.5em;
			max-width: 80%;
			max-width: calc(100% - 2em - 26px);
			overflow: auto;
			padding: 0 1em;
			padding-left: 42px;
			padding-left: calc(1em + 26px);
			background: inherit;
			background-color: #f7f8f9;
			z-index: 1;
			box-shadow: -.1em 0 .25em rgba(0,0,0,.1) inset;
		}
		body.toc-sidebar #toc h2 {
			margin-top: .8rem;
			font-variant: small-caps;
			font-variant: all-small-caps;
			text-transform: lowercase;
			font-weight: bold;
			color: gray;
			color: hsla(203,20%,40%,.7);
		}
		body.toc-sidebar #toc-jump:not(:focus) {
			width: 0;
			height: 0;
			padding: 0;
			position: absolute;
			overflow: hidden;
		}
	}
	/* Hide main scroller when only the ToC is visible anyway */
	@media screen and (max-width: 28em) {
		body.toc-sidebar {
			overflow: hidden;
		}
	}

	/* Sidebar with its own space */
	@media screen and (min-width: 78em) {
		body:not(.toc-inline) #toc {
			position: fixed;
			top: 0; bottom: 0;
			left: 0;
			width: 23.5em;
			overflow: auto;
			padding: 0 1em;
			padding-left: 42px;
			padding-left: calc(1em + 26px);
			background: inherit;
			background-color: #f7f8f9;
			z-index: 1;
			box-shadow: -.1em 0 .25em rgba(0,0,0,.1) inset;
		}
		body:not(.toc-inline) #toc h2 {
			margin-top: .8rem;
			font-variant: small-caps;
			font-variant: all-small-caps;
			text-transform: lowercase;
			font-weight: bold;
			color: gray;
			color: hsla(203,20%,40%,.7);
		}

		body:not(.toc-inline) {
			padding-left: 29em;
		}
		/* See also Overflow section at the bottom */

		body:not(.toc-inline) #toc-jump:not(:focus) {
			width: 0;
			height: 0;
			padding: 0;
			position: absolute;
			overflow: hidden;
		}
	}
	@media screen and (min-width: 90em) {
		body:not(.toc-inline) {
			margin: 0 4em;
		}
	}

/******************************************************************************/
/*                                Sectioning                                  */
/******************************************************************************/

/** Headings ******************************************************************/

	h1, h2, h3, h4, h5, h6, dt {
		page-break-after: avoid;
		page-break-inside: avoid;
		font: 100% sans-serif;   /* Reset all font styling to clear out UA styles */
		font-family: inherit;    /* Inherit the font family. */
		line-height: 1.2;        /* Keep wrapped headings compact */
		hyphens: manual;         /* Hyphenated headings look weird */
	}

	h2, h3, h4, h5, h6 {
		margin-top: 3rem;
	}

	h1, h2, h3 {
		color: #005A9C;
		background: transparent;
	}

	h1 { font-size: 170%; }
	h2 { font-size: 140%; }
	h3 { font-size: 120%; }
	h4 { font-weight: bold; }
	h5 { font-style: italic; }
	h6 { font-variant: small-caps; }
	dt { font-weight: bold; }

/** Subheadings ***************************************************************/

	h1 + h2,
	#subtitle {
		/* #subtitle is a subtitle in an H2 under the H1 */
		margin-top: 0;
	}
	h2 + h3,
	h3 + h4,
	h4 + h5,
	h5 + h6 {
		margin-top: 1.2em; /* = 1 x line-height */
	}

/** Section divider ***********************************************************/

	:not(.head) > hr {
		font-size: 1.5em;
		text-align: center;
		margin: 1em auto;
		height: auto;
		border: transparent solid 0;
		background: transparent;
	}
	:not(.head) > hr::before {
		content: "\2727\2003\2003\2727\2003\2003\2727";
	}

/******************************************************************************/
/*                            Paragraphs and Lists                            */
/******************************************************************************/

	p {
		margin: 1em 0;
	}

	dd > p:first-child,
	li > p:first-child {
		margin-top: 0;
	}

	ul, ol {
		margin-left: 0;
		padding-left: 2em;
	}

	li {
		margin: 0.25em 0 0.5em;
		padding: 0;
	}

	dl dd {
		margin: 0 0 .5em 2em;
	}

	.head dd + dd { /* compact for header */
		margin-top: -.5em;
	}

	/* Style for algorithms */
	ol.algorithm ol:not(.algorithm),
	.algorithm > ol ol:not(.algorithm) {
	 border-left: 0.5em solid #DEF;
	}

	/* Put nice boxes around each algorithm. */
	[data-algorithm]:not(.heading) {
	  padding: .5em;
	  border: thin solid #ddd; border-radius: .5em;
	  margin: .5em calc(-0.5em - 1px);
	}
	[data-algorithm]:not(.heading) > :first-child {
	  margin-top: 0;
	}
	[data-algorithm]:not(.heading) > :last-child {
	  margin-bottom: 0;
	}

	/* Style for switch/case <dl>s */
	dl.switch > dd > ol.only,
	dl.switch > dd > .only > ol {
	 margin-left: 0;
	}
	dl.switch > dd > ol.algorithm,
	dl.switch > dd > .algorithm > ol {
	 margin-left: -2em;
	}
	dl.switch {
	 padding-left: 2em;
	}
	dl.switch > dt {
	 text-indent: -1.5em;
	 margin-top: 1em;
	}
	dl.switch > dt + dt {
	 margin-top: 0;
	}
	dl.switch > dt::before {
	 content: '\21AA';
	 padding: 0 0.5em 0 0;
	 display: inline-block;
	 width: 1em;
	 text-align: right;
	 line-height: 0.5em;
	}

/** Terminology Markup ********************************************************/


/******************************************************************************/
/*                                 Inline Markup                              */
/******************************************************************************/

/** Terminology Markup ********************************************************/
	dfn   { /* Defining instance */
		font-weight: bolder;
	}
	a > i { /* Instance of term */
		font-style: normal;
	}
	dt dfn code, code.idl {
		font-size: medium;
	}
	dfn var {
		font-style: normal;
	}

/** Change Marking ************************************************************/

	del { color: red;  text-decoration: line-through; }
	ins { color: #080; text-decoration: underline;    }

/** Miscellaneous improvements to inline formatting ***************************/

	sup {
		vertical-align: super;
		font-size: 80%
	}

/******************************************************************************/
/*                                    Code                                    */
/******************************************************************************/

/** General monospace/pre rules ***********************************************/

	pre, code, samp {
		font-family: Menlo, Consolas, "DejaVu Sans Mono", Monaco, monospace;
		font-size: .9em;
		page-break-inside: avoid;
		hyphens: none;
		text-transform: none;
	}
	pre code,
	code code {
		font-size: 100%;
	}

	pre {
		margin-top: 1em;
		margin-bottom: 1em;
		overflow: auto;
	}

/** Inline Code fragments *****************************************************/

  /* Do something nice. */

/******************************************************************************/
/*                                    Links                                   */
/******************************************************************************/

/** General Hyperlinks ********************************************************/

	/* We hyperlink a lot, so make it less intrusive */
	a[href] {
		color: #034575;
		text-decoration: none;
		border-bottom: 1px solid #707070;
		/* Need a bit of extending for it to look okay */
		padding: 0 1px 0;
		margin: 0 -1px 0;
	}
	a:visited {
		border-bottom-color: #BBB;
	}

	/* Use distinguishing colors when user is interacting with the link */
	a[href]:focus,
	a[href]:hover {
		background: #f8f8f8;
		background: rgba(75%, 75%, 75%, .25);
		border-bottom-width: 3px;
		margin-bottom: -2px;
	}
	a[href]:active {
		color: #C00;
		border-color: #C00;
	}

	/* Backout above styling for W3C logo */
	.head .logo,
	.head .logo a {
		border: none;
		text-decoration: none;
		background: transparent;
	}

/******************************************************************************/
/*                                    Images                                  */
/******************************************************************************/

	img {
		border-style: none;
	}

	/* For autogen numbers, add
	   .caption::before, figcaption::before { content: "Figure " counter(figure) ". "; }
	*/

	figure, .figure, .sidefigure {
		page-break-inside: avoid;
		text-align: center;
		margin: 2.5em 0;
	}
	.figure img,    .sidefigure img,    figure img,
	.figure object, .sidefigure object, figure object {
		max-width: 100%;
		margin: auto;
	}
	.figure pre, .sidefigure pre, figure pre {
		text-align: left;
		display: table;
		margin: 1em auto;
	}
	.figure table, figure table {
		margin: auto;
	}
	@media screen and (min-width: 20em) {
		.sidefigure {
			float: right;
			width: 50%;
			margin: 0 0 0.5em 0.5em
		}
	}
	.caption, figcaption, caption {
		font-style: italic;
		font-size: 90%;
	}
	.caption::before, figcaption::before, figcaption > .marker {
		font-weight: bold;
	}
	.caption, figcaption {
		counter-increment: figure;
	}

	/* DL list is indented 2em, but figure inside it is not */
	dd > .figure, dd > figure { margin-left: -2em }

/******************************************************************************/
/*                             Colored Boxes                                  */
/******************************************************************************/

	.issue, .note, .example, .assertion, .advisement, blockquote {
		padding: .5em;
		border: .5em;
		border-left-style: solid;
		page-break-inside: avoid;
	}
	span.issue, span.note {
		padding: .1em .5em .15em;
		border-right-style: solid;
	}

	.issue,
	.note,
	.example,
	.advisement,
	.assertion,
	blockquote {
		margin: 1em auto;
	}
	.note  > p:first-child,
	.issue > p:first-child,
	blockquote > :first-child {
		margin-top: 0;
	}
	blockquote > :last-child {
		margin-bottom: 0;
	}

/** Blockquotes ***************************************************************/

	blockquote {
		border-color: silver;
	}

/** Open issue ****************************************************************/

	.issue {
		border-color: #E05252;
		background: #FBE9E9;
		counter-increment: issue;
		overflow: auto;
	}
	.issue::before, .issue > .marker {
		text-transform: uppercase;
		color: #AE1E1E;
		padding-right: 1em;
		text-transform: uppercase;
	}
	/* Add .issue::before { content: "Issue " counter(issue) " "; } for autogen numbers,
	   or use class="marker" to mark up the issue number in source. */

/** Example *******************************************************************/

	.example {
		border-color: #E0CB52;
		background: #FCFAEE;
		counter-increment: example;
		overflow: auto;
		clear: both;
	}
	.example::before, .example > .marker {
		text-transform: uppercase;
		color: #827017;
		min-width: 7.5em;
		display: block;
	}
	/* Add .example::before { content: "Example " counter(example) " "; } for autogen numbers,
	   or use class="marker" to mark up the example number in source. */

/** Non-normative Note ********************************************************/

	.note {
		border-color: #52E052;
		background: #E9FBE9;
		overflow: auto;
	}

	.note::before, .note > .marker,
	details.note > summary::before,
	details.note > summary > .marker {
		text-transform: uppercase;
		display: block;
		color: hsl(120, 70%, 30%);
	}
	/* Add .note::before { content: "Note"; } for autogen label,
	   or use class="marker" to mark up the label in source. */

	details.note > summary {
		display: block;
		color: hsl(120, 70%, 30%);
	}
	details.note[open] > summary {
		border-bottom: 1px silver solid;
	}

/** Assertion Box *************************************************************/
	/*  for assertions in algorithms */

	.assertion {
		border-color: #AAA;
		background: #EEE;
	}

/** Advisement Box ************************************************************/
	/*  for attention-grabbing normative statements */

	.advisement {
		border-color: orange;
		border-style: none solid;
		background: #FFEECC;
	}
	strong.advisement {
		display: block;
		text-align: center;
	}
	.advisement > .marker {
		color: #B35F00;
	}

/** Spec Obsoletion Notice ****************************************************/
	/* obnoxious obsoletion notice for older/abandoned specs. */

	details {
		display: block;
	}
	summary {
		font-weight: bolder;
	}

	.annoying-warning:not(details),
	details.annoying-warning:not([open]) > summary,
	details.annoying-warning[open] {
		background: #fdd;
		color: red;
		font-weight: bold;
		padding: .75em 1em;
		border: thick red;
		border-style: solid;
		border-radius: 1em;
	}
	.annoying-warning :last-child {
		margin-bottom: 0;
	}

@media not print {
	details.annoying-warning[open] {
		position: fixed;
		left: 1em;
		right: 1em;
		bottom: 1em;
		z-index: 1000;
	}
}

	details.annoying-warning:not([open]) > summary {
		text-align: center;
	}

/** Entity Definition Boxes ***************************************************/

	.def {
		padding: .5em 1em;
		background: #DEF;
		margin: 1.2em 0;
		border-left: 0.5em solid #8CCBF2;
	}

/******************************************************************************/
/*                                    Tables                                  */
/******************************************************************************/

	th, td {
		text-align: left;
		text-align: start;
	}

/** Property/Descriptor Definition Tables *************************************/

	table.def {
		/* inherits .def box styling, see above */
		width: 100%;
		border-spacing: 0;
	}

	table.def td,
	table.def th {
		padding: 0.5em;
		vertical-align: baseline;
		border-bottom: 1px solid #bbd7e9;
	}

	table.def > tbody > tr:last-child th,
	table.def > tbody > tr:last-child td {
		border-bottom: 0;
	}

	table.def th {
		font-style: italic;
		font-weight: normal;
		padding-left: 1em;
		width: 3em;
	}

	/* For when values are extra-complex and need formatting for readability */
	table td.pre {
		white-space: pre-wrap;
	}

	/* A footnote at the bottom of a def table */
	table.def           td.footnote {
		padding-top: 0.6em;
	}
	table.def           td.footnote::before {
		content: " ";
		display: block;
		height: 0.6em;
		width: 4em;
		border-top: thin solid;
	}

/** Data tables (and properly marked-up index tables) *************************/
	/*
		 <table class="data"> highlights structural relationships in a table
		 when correct markup is used (e.g. thead/tbody, th vs. td, scope attribute)

		 Use class="complex data" for particularly complicated tables --
		 (This will draw more lines: busier, but clearer.)

		 Use class="long" on table cells with paragraph-like contents
		 (This will adjust text alignment accordingly.)
		 Alternately use class="longlastcol" on tables, to have the last column assume "long".
	*/

	table {
		word-wrap: normal;
		overflow-wrap: normal;
		hyphens: manual;
	}

	table.data,
	table.index {
		margin: 1em auto;
		border-collapse: collapse;
		border: hidden;
		width: 100%;
	}
	table.data caption,
	table.index caption {
		max-width: 50em;
		margin: 0 auto 1em;
	}

	table.data td,  table.data th,
	table.index td, table.index th {
		padding: 0.5em 1em;
		border-width: 1px;
		border-color: silver;
		border-top-style: solid;
	}

	table.data thead td:empty {
		padding: 0;
		border: 0;
	}

	table.data  thead,
	table.index thead,
	table.data  tbody,
	table.index tbody {
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/******************************************************************************/
/*                                  Indices                                   */
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/*                             Overflow Control                               */
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 <body class="h-entry">
  <div class="head">
   <p data-fill-with="logo"></p>
   <h1 class="p-name no-ref" id="title">P0690R0<br>Tearable Atomics</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Published Proposal, <time class="dt-updated" datetime="1970-01-01">1970-01-01</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="http://wg21.link/P0690">http://wg21.link/P0690</a>
     <dt>Authors:
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:jfbastien@apple.com">JF Bastien</a> (<span class="p-org org">Apple</span>)
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:bion@microsoft.com">Billy Robert O'Neal III</a> (<span class="p-org org">Microsoft</span>)
     <dt>Audience:
     <dd>SG1
     <dt>Project:
     <dd>ISO/IEC JTC1/SC22/WG21 14882: Programming Language — C++
     <dt>Source:
     <dd><a href="https://github.com/jfbastien/papers/blob/master/source/P0690r0.bs">github.com/jfbastien/papers/blob/master/source/P0690r0.bs</a>
    </dl>
   </div>
   <div data-fill-with="warning"></div>
   <hr title="Separator for header">
  </div>
  <div class="p-summary" data-fill-with="abstract">
   <h2 class="no-num no-toc no-ref heading settled" id="abstract"><span class="content">Abstract</span></h2>
   <p>Atomics which can tear—which are more relaxed than relaxed—seem useless. This paper shows otherwise.</p>
  </div>
  <nav data-fill-with="table-of-contents" id="toc">
   <h2 class="no-num no-toc no-ref" id="contents">Table of Contents</h2>
   <ol class="toc" role="directory">
    <li><a href="#bg"><span class="secno">1</span> <span class="content">Background</span></a>
    <li>
     <a href="#usecases"><span class="secno">2</span> <span class="content">Usecases</span></a>
     <ol class="toc">
      <li><a href="#spec-load"><span class="secno">2.1</span> <span class="content">Speculative Load</span></a>
      <li><a href="#seqlock"><span class="secno">2.2</span> <span class="content">Seqlock</span></a>
      <li><a href="#wsdeque"><span class="secno">2.3</span> <span class="content">Work-Stealing Deque</span></a>
     </ol>
    <li><a href="#moar"><span class="secno">3</span> <span class="content">Further Considerations</span></a>
    <li><a href="#solutions"><span class="secno">4</span> <span class="content">Solutions</span></a>
    <li>
     <a href="#references"><span class="secno"></span> <span class="content">References</span></a>
     <ol class="toc">
      <li><a href="#informative"><span class="secno"></span> <span class="content">Informative References</span></a>
     </ol>
   </ol>
  </nav>
  <main>
   <h2 class="heading settled" data-level="1" id="bg"><span class="secno">1. </span><span class="content">Background</span><a class="self-link" href="#bg"></a></h2>
   <p>Is it useful for C++ to support "tearable" atomic memory ordering, where the
access participates in atomic ordering as strongly as <code class="highlight"><c- n>memory_order_relaxed</c-></code> accesses, but where the memory is allowed to tear (i.e. isn’t single-copy
atomic). In C++ standards speak: particular atomic object are <strong>not</strong> indivisible with respect to all other atomic accesses to that object.</p>
   <p>Indeed, advanced concurrency and parallelism users will sometimes find a need
for objects which are accessed by multiple threads, yet either:</p>
   <ol>
    <li data-md>
     <p>Rely on separate atomic objects to provide inter-thread observability
 guarantees; or</p>
    <li data-md>
     <p>Use lock-free accesses on a memory locations on which they would also like
 to speculate.</p>
   </ol>
   <p>What we describe often amounts to a <em>data race</em>:</p>
   <ul>
    <li data-md>
     <p>Two accesses to the same <em>memory location</em> by different threads are <em>not
ordered</em></p>
    <li data-md>
     <p>At least one of them stores to the memory location</p>
    <li data-md>
     <p>At least one of them is not a synchronization action</p>
   </ul>
   <p>Data races are <em>undefined behavior</em>, and it is often said that there is no such
thing as a "benign" data race <a data-link-type="biblio" href="#biblio-benign">[benign]</a>. Issues that arise when mixing atomic
and non-atomic accesses have been discussed before in the context of the C
language’s memory model <a data-link-type="biblio" href="#biblio-n4136">[n4136]</a>.</p>
   <p>Reconciling these types of issue is discussed in the concurrency and parallelism
group from time to time, and so far only one-off solutions have been proposed to
the Committee, or the problem has been punted. We believe that this proposal can
fix this interesting problem once and for all by looking at how other areas have
solved this issue.</p>
   <p>After all, this has been solved in non-C++ contexts. To assembly programmers, or
to those used to memory models such as Linux’s memory model <a data-link-type="biblio" href="#biblio-p0124r2">[p0124r2]</a>, the
distinctions the Standard makes seems overly complex. Their code simply defines
atomicity as a property or <em>code</em> rather than C++'s definition of atomicity as a
property of <em>particular memory locations</em> during an object’s lifetime. Indeed,
in assembly a memory location can be concurrently accessed with a regular
non-atomic memory instruction as well as an atomic memory instruction.</p>
   <h2 class="heading settled" data-level="2" id="usecases"><span class="secno">2. </span><span class="content">Usecases</span><a class="self-link" href="#usecases"></a></h2>
   <p>Sample usecases include:</p>
   <ol>
    <li data-md>
     <p>Speculative load before a compare-and-exchange instruction</p>
    <li data-md>
     <p>Sequence locks</p>
    <li data-md>
     <p>Work-stealing deque</p>
   </ol>
   <p>Others exist, but we will focus on these three.</p>
   <h3 class="heading settled" data-level="2.1" id="spec-load"><span class="secno">2.1. </span><span class="content">Speculative Load</span><a class="self-link" href="#spec-load"></a></h3>
   <p>Consider the following code:</p>
<pre class="highlight"><c- k>struct</c-> <c- nf>alignas</c-><c- p>(</c-><c- k>sizeof</c-><c- p>(</c-><c- b>intptr_t</c-><c- p>)</c-> <c- o>*</c-> <c- mi>2</c-><c- p>)</c-> <c- n>Node</c-> <c- p>{</c->
    <c- b>intptr_t</c-> <c- n>l</c-><c- p>,</c-> <c- n>r</c-><c- p>;</c->
<c- p>};</c->

<c- n>Node</c-> <c- nf>action</c-><c- p>(</c-><c- k>const</c-> <c- n>Node</c-><c- o>&amp;</c-> <c- n>old</c-><c- p>);</c->

<c- b>void</c-> <c- nf>do_action</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>atomic</c-><c- o>&lt;</c-><c- n>Node</c-><c- o>></c-> <c- o>*</c-><c- n>n</c-><c- p>)</c-> <c- p>{</c->
    <c- n>Node</c-> <c- n>old</c-> <c- o>=</c-> <c- n>n</c-><c- o>-></c-><c- n>load</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>memory_order_relaxed</c-><c- p>);</c-> <c- c1>// Relaxed loads can’t tear.</c->
    <c- k>while</c-> <c- p>(</c-><c- o>!</c-><c- n>n</c-><c- o>-></c-><c- n>compare_exchange_weak</c-><c- p>(</c-><c- n>old</c-><c- p>,</c-> <c- n>action</c-><c- p>(</c-><c- n>old</c-><c- p>),</c-> <c- n>std</c-><c- o>::</c-><c- n>memory_order_release</c-><c- p>,</c->
                                     <c- n>std</c-><c- o>::</c-><c- n>memory_order_acquire</c-><c- p>))</c->
      <c- p>;</c->
<c- p>}</c->
<c- p>};</c->
</pre>
   <p>In this example, all lock-free operations (including load / store) <em>must</em> be
implemented as a compare-and-exchange or load-linked / store-conditional:</p>
   <ul>
    <li data-md>
     <p>On recent x86-64 using <code class="highlight"><c- n>cmpxchg16b</c-></code>.</p>
    <li data-md>
     <p>On A32 without LPAE using <code class="highlight"><c- n>ldrexd</c-></code>, <code class="highlight"><c- n>clrex</c-></code>, and <code class="highlight"><c- n>strexd</c-></code>.</p>
   </ul>
   <p>The relaxed memory access could instead speculate by using a tearable load /
store, potentially cheaper than a compare-and-exchange or load-link /
store-conditional, as long as a compare-and-exchange retry loop follows it to
handle races. If tearing occurs then the compare-and-exchange does the right
thing.</p>
   <ul>
    <li data-md>
     <p>On x86-64 using two <code class="highlight"><c- n>movq</c-></code> instructions (two instructions are never locked
and can tear).</p>
    <li data-md>
     <p>On A32 using <code class="highlight"><c- n>ldrd</c-></code> (without LPAE the instruction isn’t single-copy atomic).</p>
   </ul>
   <h3 class="heading settled" data-level="2.2" id="seqlock"><span class="secno">2.2. </span><span class="content">Seqlock</span><a class="self-link" href="#seqlock"></a></h3>
   <p>In the case of sequence locks, the data being protected can be accessed
non-atomically and is known to be race-free if the sequence number hasn’t
changed before and after the data was retrieved, and if it isn’t "tagged" as
being modified (below, by being odd):</p>
<pre class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>typename</c-> <c- n>T</c-><c- o>></c->
<c- k>struct</c-> <c- n>Data</c-> <c- p>{</c->
  <c- n>std</c-><c- o>::</c-><c- n>atomic</c-><c- o>&lt;</c-><c- b>unsigned</c-><c- o>></c-> <c- n>sequence_number</c-> <c- o>=</c-> <c- mi>0</c-><c- p>;</c->
  <c- n>std</c-><c- o>::</c-><c- n>atomic</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-> <c- n>value0</c-><c- p>;</c->
  <c- n>std</c-><c- o>::</c-><c- n>atomic</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-> <c- n>value1</c-><c- p>;</c->
<c- p>};</c->

<c- n>std</c-><c- o>::</c-><c- n>tuple</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>T</c-><c- o>></c-> <c- n>reader</c-><c- p>(</c-><c- k>const</c-> <c- n>Data</c-><c- o>&amp;</c-> <c- n>data</c-><c- p>)</c-> <c- p>{</c->
  <c- n>T</c-> <c- n>value0</c-><c- p>,</c-> <c- n>value1</c-><c- p>;</c->
  <c- b>unsigned</c-> <c- n>sequence_before</c-><c- p>,</c-> <c- n>sequence_after</c-><c- p>;</c->
  <c- k>do</c-> <c- p>{</c->
    <c- n>sequence_before</c-> <c- o>=</c-> <c- n>data</c-><c- p>.</c-><c- n>sequence_number</c-><c- p>.</c-><c- n>load</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>memory_order_acquire</c-><c- p>);</c->
    <c- n>value0</c-> <c- o>=</c-> <c- n>data</c-><c- p>.</c-><c- n>value0</c-><c- p>.</c-><c- n>load</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>memory_order_relaxed</c-><c- p>);</c->
    <c- n>value1</c-> <c- o>=</c-> <c- n>data</c-><c- p>.</c-><c- n>value1</c-><c- p>.</c-><c- n>load</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>memory_order_relaxed</c-><c- p>);</c->
    <c- n>std</c-><c- o>::</c-><c- n>atomic_thread_fence</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>memory_order_acquire</c-><c- p>);</c->
    <c- n>sequence_after</c-> <c- o>=</c-> <c- n>data</c-><c- p>.</c-><c- n>sequence_number</c-><c- p>.</c-><c- n>load</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>memory_order_relaxed</c-><c- p>);</c->
  <c- p>}</c-> <c- k>while</c-> <c- p>(</c-><c- n>sequence_before</c-> <c- o>!=</c-> <c- n>sequence_after</c-> <c- o>||</c-> <c- n>sequence_before</c-> <c- o>&amp;</c-> <c- mi>1</c-><c- p>);</c->
  <c- k>return</c-> <c- p>{</c-><c- n>value0</c-><c- p>,</c-> <c- n>value1</c-><c- p>};</c->
<c- p>}</c->

<c- b>void</c-> <c- n>writer</c-><c- p>(</c-><c- n>Data</c-><c- o>&amp;</c-> <c- n>data</c-><c- p>,</c-> <c- n>T</c-> <c- n>value0</c-><c- p>,</c-> <c- n>T</c-> <c- n>value1</c-><c- p>)</c-> <c- p>{</c->
  <c- k>auto</c-> <c- n>sequence_start</c-> <c- o>=</c-> <c- n>data</c-><c- p>.</c-><c- n>sequence_number</c-><c- p>.</c-><c- n>load</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>memory_order_relaxed</c-><c- p>);</c->
  <c- n>data</c-><c- p>.</c-><c- n>sequence_number</c-><c- p>.</c-><c- n>store</c-><c- p>(</c-><c- n>sequence_start</c-> <c- o>+</c-> <c- mi>1</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>memory_order_relaxed</c-><c- p>);</c->
  <c- n>data</c-><c- p>.</c-><c- n>value0</c-><c- p>.</c-><c- n>store</c-><c- p>(</c-><c- n>value0</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>memory_order_release</c-><c- p>);</c->
  <c- n>data</c-><c- p>.</c-><c- n>value1</c-><c- p>.</c-><c- n>store</c-><c- p>(</c-><c- n>value1</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>memory_order_release</c-><c- p>);</c->
  <c- n>data</c-><c- p>.</c-><c- n>sequence_number</c-><c- p>.</c-><c- n>store</c-><c- p>(</c-><c- n>sequence_start</c-> <c- o>+</c-> <c- mi>2</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>memory_order_release</c-><c- p>);</c->
<c- p>}</c->
</pre>
   <p>Notice that in C++ the values being protected must be atomic because this
algorithm doesn’t use more common acquire / release patterns which C++
encourages. Doing otherwise would be a data race according to the memory
model. One would need to add fences for non-atomic accesses to not be racy.</p>
   <p>A more in-depth discussion of seqlock <a data-link-type="biblio" href="#biblio-seqlock">[seqlock]</a> is available.</p>
   <p>For the purpose of our discussion, it is especially interesting to considers
value types <code class="highlight"><c- n>T</c-></code> which are never lock-free.</p>
   <h3 class="heading settled" data-level="2.3" id="wsdeque"><span class="secno">2.3. </span><span class="content">Work-Stealing Deque</span><a class="self-link" href="#wsdeque"></a></h3>
   <p>It appears intended that implementations of the parallelism TS <a data-link-type="biblio" href="#biblio-n4578">[N4578]</a> back
scheduling of partitioned work using an ABP work-stealing scheduler, originally
described in <a data-link-type="biblio" href="#biblio-threadsched">[ThreadSched]</a>.</p>
   <p>Under this model, each thread has a local deque of work where it can access the
"bottom" of the deque without any synchronization overhead, but other threads
can concurrently remove work from the "top". A similar data structure was
presented implementable on hardware without double-wide CAS instructions in <a data-link-type="biblio" href="#biblio-wsdeque">[WSDeque]</a>.</p>
   <p>In the Chase-Lev deque, the "top" counter is used to track concurrent access to
the top of the deque; and access to the actual elements in the top of the deque
is unsynchronized. From the original paper:</p>
<pre class="highlight line-numbered" line-start="10"><span class="line-no" data-line="10"></span><span class="line"><c- k>public</c-> <c- n>Object</c-> <c- nf>steal</c-><c- p>()</c-> <c- p>{</c-></span><span class="line-no" data-line="11"></span><span class="line">    <c- b>long</c-> <c- n>t</c-> <c- o>=</c-> <c- k>this</c-><c- p>.</c-><c- n>top</c-><c- p>;</c-></span><span class="line-no" data-line="12"></span><span class="line">    <c- b>long</c-> <c- n>b</c-> <c- o>=</c-> <c- k>this</c-><c- p>.</c-><c- n>bottom</c-><c- p>;</c-></span><span class="line-no" data-line="13"></span><span class="line">    <c- n>CircularArray</c-> <c- n>a</c-> <c- o>=</c-> <c- k>this</c-><c- p>.</c-><c- n>activeArray</c-><c- p>;</c-></span><span class="line-no" data-line="14"></span><span class="line">    <c- b>long</c-> <c- n>size</c-> <c- o>=</c-> <c- n>b</c-> <c- o>-</c-> <c- n>t</c-><c- p>;</c-></span><span class="line-no" data-line="15"></span><span class="line">    <c- k>if</c-> <c- p>(</c-><c- n>size</c-> <c- o>&lt;=</c-> <c- mi>0</c-><c- p>)</c-> <c- k>return</c-> <c- n>Empty</c-><c- p>;</c-></span><span class="line-no" data-line="16"></span><span class="line">    <c- n>Object</c-> <c- n>o</c-> <c- o>=</c-> <c- n>a</c-><c- p>.</c-><c- n>get</c-><c- p>(</c-><c- n>t</c-><c- p>);</c-></span><span class="line-no" data-line="17"></span><span class="line">    <c- k>if</c-> <c- p>(</c-><c- o>!</c-> <c- n>casTop</c-><c- p>(</c-><c- n>t</c-><c- p>,</c-> <c- n>t</c-><c- o>+</c-><c- mi>1</c-><c- p>))</c-></span><span class="line-no" data-line="18"></span><span class="line">        <c- k>return</c-> <c- n>Abort</c-><c- p>;</c-></span><span class="line-no" data-line="19"></span><span class="line">    <c- k>return</c-> <c- n>o</c-><c- p>;</c-></span><span class="line-no" data-line="20"></span><span class="line"><c- p>}</c-></span></pre>
   <p>or translated to C++:</p>
<pre class="highlight line-numbered" line-start="10"><span class="line-no" data-line="10"></span><span class="line"><c- n>variant</c-><c- o>&lt;</c-><c- n>empty_t</c-><c- p>,</c-> <c- n>abort_t</c-><c- p>,</c-> <c- n>T</c-><c- o>></c-> <c- n>steal</c-><c- p>()</c-> <c- p>{</c-></span><span class="line-no" data-line="11"></span><span class="line">    <c- b>int64_t</c-> <c- n>t</c-> <c- o>=</c-> <c- n>top</c-><c- p>.</c-><c- n>load</c-><c- p>();</c-></span><span class="line-no" data-line="12"></span><span class="line">    <c- b>int64_t</c-> <c- n>b</c-> <c- o>=</c-> <c- n>bottom</c-><c- p>.</c-><c- n>load</c-><c- p>();</c-></span><span class="line-no" data-line="13"></span><span class="line">    <c- n>T</c-> <c- o>*</c-> <c- n>a</c-> <c- o>=</c-> <c- n>activeArray</c-><c- p>.</c-><c- n>load</c-><c- p>();</c-></span><span class="line-no" data-line="14"></span><span class="line">    <c- b>int64_t</c-> <c- n>size</c-> <c- o>=</c-> <c- n>b</c-> <c- o>-</c-> <c- n>t</c-><c- p>;</c-></span><span class="line-no" data-line="15"></span><span class="line">    <c- k>if</c-> <c- p>(</c-><c- n>size</c-> <c- o>&lt;=</c-> <c- mi>0</c-><c- p>)</c-> <c- k>return</c-> <c- n>empty_t</c-><c- p>{};</c-></span><span class="line-no" data-line="16"></span><span class="line">    <c- n>T</c-> <c- n>o</c-> <c- o>=</c-> <c- n>a</c-><c- p>[</c-><c- n>t</c-> <c- o>%</c-> <c- n>aSize</c-><c- p>];</c-></span><span class="line-no" data-line="17"></span><span class="line">    <c- k>if</c-> <c- p>(</c-><c- o>!</c-> <c- n>top</c-><c- p>.</c-><c- n>compare_exchange_strong</c-><c- p>(</c-><c- n>t</c-><c- p>,</c-> <c- n>t</c-><c- o>+</c-><c- mi>1</c-><c- p>))</c-></span><span class="line-no" data-line="18"></span><span class="line">        <c- k>return</c-> <c- n>abort_t</c-><c- p>{};</c-></span><span class="line-no" data-line="19"></span><span class="line">    <c- k>return</c-> <c- n>o</c-><c- p>;</c-></span><span class="line-no" data-line="20"></span><span class="line"><c- p>}</c-></span></pre>
   <p>If the deque contains only one element, this causes undefined behavior in C++'s
memory model, because the element accessed on line 16 may be concurrently
written by the owning thread of the deque. However, if a data race occurs, the
CAS on line 17 fails, and the result of this speculative read is never
observed. Only one thread can win the CAS race to increment top.</p>
   <p>Of note, imposing that <code class="highlight"><c- n>T</c-></code> be an atomic type in this instance defeats the entire
purpose of using the work-stealing deque, as it would require the owning
"bottom" thread to use synchronized access to read from the bottom (including
potentially taking a lock of <code class="highlight"><c- n>T</c-></code> is large; egregious for a data structure whose
purpose is to be lock-free) in the uncontended cases, even though the
correctness of the algorithm is maintained by discarding any potentially torn
results.</p>
   <h2 class="heading settled" data-level="3" id="moar"><span class="secno">3. </span><span class="content">Further Considerations</span><a class="self-link" href="#moar"></a></h2>
   <p>Extrapolating from the above examples, it is also useful to consider a few extra
usecases where:</p>
   <ul>
    <li data-md>
     <p>Alignment of the datastructures is purposefully <em>not</em> natural. In contrast, <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>atomic</c-></code> is specified as always being suitably aligned by the
implementation.</p>
    <li data-md>
     <p>Padding of the datastructure isn’t the same as that mandated by <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>atomic</c-></code> (although padding bits are their own bag of special <a data-link-type="biblio" href="#biblio-p0528r0">[p0528r0]</a>).</p>
    <li data-md>
     <p>The datastructure isn’t always accessed by memory operations of the same
byte-size. This could occur without dangerous type aliasing by using
properly type-punned <code class="highlight"><c- k>union</c-></code>, <code class="highlight"><c- n>memcpy</c-></code>, or <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>variant</c-></code>, as well as with
SIMD types that sometimes perform element accesses.</p>
    <li data-md>
     <p>The datastructure being accessed is large, making it non-lock-free and
requiring an implementation-provided lock. Many implementations rely on lock
sharding for this, but some embed a lock in every large <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>atomic</c-></code> object.</p>
   </ul>
   <h2 class="heading settled" data-level="4" id="solutions"><span class="secno">4. </span><span class="content">Solutions</span><a class="self-link" href="#solutions"></a></h2>
   <p>There are many solutions to this problem. This paper hopes to round up what has
been suggested before, leading to a discussion in the concurrency and
parallelism group. This discussion should end in straw polls which provide
guidance on where the committee would like to go next with this issue.</p>
   <ol>
    <li data-md>
     <p>Atomic views <a data-link-type="biblio" href="#biblio-p0019r5">[p0019r5]</a> tackle some of the issues discussed here, but in
 an environment where data access patterns follow <em>epochs</em>. For parts of
 runtime the view are accessed non-atomically, and for other parts of
 runtime they are accessed atomically. Atomic views do not solve the general
 problem discussed here.</p>
    <li data-md>
     <p>A paper on thin air values <a data-link-type="biblio" href="#biblio-n3710">[n3710]</a> discussed adding <code class="highlight"><c- n>non_atomic_load</c-><c- p>()</c-></code>, <code class="highlight"><c- n>non_atomic_store</c-><c- p>()</c-></code>, and <code class="highlight"><c- n>race_or</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code> type (similar to <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>optional</c-></code> or <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>expected</c-></code> but for racy / indeterminate results).</p>
    <li data-md>
     <p>Safe <code class="highlight"><c- n>memcpy</c-></code> <a data-link-type="biblio" href="#biblio-p0603r0">[p0603r0]</a> proposes addressing the seqlock example with <code class="highlight"><c- n>nonatomic_load</c-><c- p>()</c-></code> and <code class="highlight"><c- n>nonatomic_store</c-><c- p>()</c-></code> functions.</p>
    <li data-md>
     <p>We also offer a different approach: a new memory order type, <code class="highlight"><c- n>memory_order_tearing</c-></code>, which has the same semantics as <code class="highlight"><c- n>memory_order_relaxed</c-></code> but which is allowed to tear. And—of course—<wbr><code class="highlight"><c- n>memory_order_tearing</c-></code> has the neat properly of being spelled with the same
 number of characters as the other 6 memory orderings.</p>
   </ol>
   <p>Not all of these approaches address all the issues discussed
above—e.g. <code class="highlight"><c- n>memory_order_tearing</c-></code> does not address the issue of large
non-lock-free <code class="highlight"><c- n>T</c-></code>—we therefore hope that the concurency and parallelism group
will find the wisdom required to weigh each issue and decide which solution fits
them best.</p>
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  <h2 class="no-num no-ref heading settled" id="references"><span class="content">References</span><a class="self-link" href="#references"></a></h2>
  <h3 class="no-num no-ref heading settled" id="informative"><span class="content">Informative References</span><a class="self-link" href="#informative"></a></h3>
  <dl>
   <dt id="biblio-benign">[BENIGN]
   <dd>Hans-J. Boehm. <a href="http://hboehm.info/boehm-hotpar11.pdf">How to miscompile programs with “benign” data races</a>. May 2011. URL: <a href="http://hboehm.info/boehm-hotpar11.pdf">http://hboehm.info/boehm-hotpar11.pdf</a>
   <dt id="biblio-n3710">[N3710]
   <dd>H. Boehm, et al.. <a href="https://wg21.link/n3710">Specifying the absence of "out of thin air" results (LWG2265)</a>. URL: <a href="https://wg21.link/n3710">https://wg21.link/n3710</a>
   <dt id="biblio-n4136">[N4136]
   <dd>M. Batty, P. Sewell, et al.. <a href="https://wg21.link/n4136">C Concurrency Challenges Draft</a>. 13 October 2014. URL: <a href="https://wg21.link/n4136">https://wg21.link/n4136</a>
   <dt id="biblio-n4578">[N4578]
   <dd>Jared Hoberock. <a href="https://wg21.link/n4578">Working Draft, Technical Specification for C++ Extensions for Parallelism Version 2</a>. 22 February 2016. URL: <a href="https://wg21.link/n4578">https://wg21.link/n4578</a>
   <dt id="biblio-p0019r5">[P0019R5]
   <dd>H. Carter Edwards, Hans Boehm, Olivier Giroux, James Reus. <a href="https://wg21.link/p0019r5">Atomic View</a>. 6 March 2017. URL: <a href="https://wg21.link/p0019r5">https://wg21.link/p0019r5</a>
   <dt id="biblio-p0124r2">[P0124R2]
   <dd>Paul E. McKenney, Ulrich Weigand, Andrea Parri, Boqun Feng. <a href="https://wg21.link/p0124r2">Linux-Kernel Memory Model</a>. 26 June 2016. URL: <a href="https://wg21.link/p0124r2">https://wg21.link/p0124r2</a>
   <dt id="biblio-p0528r0">[P0528R0]
   <dd>JF Bastien, Michael Spencer. <a href="https://wg21.link/p0528r0">The Curious Case of Padding Bits, Featuring Atomic Compare-and-Exchange</a>. 12 November 2016. URL: <a href="https://wg21.link/p0528r0">https://wg21.link/p0528r0</a>
   <dt id="biblio-p0603r0">[P0603R0]
   <dd>Andrew Hunter. <a href="https://wg21.link/p0603r0">safe memcpy: A simpler implementation primitive for seqlock and friends</a>. 17 March 2017. URL: <a href="https://wg21.link/p0603r0">https://wg21.link/p0603r0</a>
   <dt id="biblio-seqlock">[SEQLOCK]
   <dd>Hans-J. Boehm. <a href="http://safari.ece.cmu.edu/MSPC2012/slides_posters/boehm-slides.pdf">Can Seqlocks Get Along with Programming Language Memory Models?</a>. 16 June 2012. URL: <a href="http://safari.ece.cmu.edu/MSPC2012/slides_posters/boehm-slides.pdf">http://safari.ece.cmu.edu/MSPC2012/slides_posters/boehm-slides.pdf</a>
   <dt id="biblio-threadsched">[ThreadSched]
   <dd>Nimar S. Arora; Robert D. Blumofe; C. Greg Plaxton. <a href="https://www.eecis.udel.edu/~cavazos/cisc879-spring2008/papers/arora98thread.pdf">Thread Scheduling for Multiprogrammed Microprocessors</a>. June 1998. URL: <a href="https://www.eecis.udel.edu/~cavazos/cisc879-spring2008/papers/arora98thread.pdf">https://www.eecis.udel.edu/~cavazos/cisc879-spring2008/papers/arora98thread.pdf</a>
   <dt id="biblio-wsdeque">[WSDeque]
   <dd>David Chase; Yossi Lev. <a href="http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.170.1097&amp;rep=rep1&amp;type=pdf">Dynamic Circular Work-Stealing Deque</a>. July 2005. URL: <a href="http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.170.1097&amp;rep=rep1&amp;type=pdf">http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.170.1097&amp;rep=rep1&amp;type=pdf</a>
  </dl>